Although Ni is the sixth most abundant element on Earth, most of it resides in the core, leaving Ni as a trace element in most crustal rocks.^1,2  The name “Nickel”, German for “demon”, allegedly originated from miners’ frustrations due to the contamination of copper sulfide deposits with nickel. While Ni catalyzes vital reactions in the carbon and nitrogen cycles in the modern era,³  it might have been even more biochemically important early in Earth’s history.⁴  In the anoxic Archean eon (>2.5 billion years ago), ultra-high temperature lavas erupted from the ocean’s crust, and through chemical dissolution supplied abundant Ni to seawater.⁵  The relative abundance of Ni enzymes in strictly anaerobic microbial metabolisms involving reduced gases (H₂, CO, and CH₄),^4,6  and its extremely limited use by eukaryotes,⁷  suggest that Ni's biological functions are largely evolutionary relicts of the anoxic Archean. Roughly a half billion years ago, geologic records suggest that seawater Ni concentrations plummeted again.⁵  Compared to the use of some metals (e.g. Fe and Zn) in hundreds of different proteins, the relative paucity of Ni enzymes in modern biology implies that evolution has likely favored the use of alternative metals in place of Ni,^8,9  although several enzymes remain solely Ni dependent, and new Ni-containing proteins continue to be discovered.¹⁰  Here we review: (i) the modern chemical cycling of Ni in different regions of the ocean; (ii) major enzymatic uses for Ni in marine microbes; (iii) the growth response of diverse microbial taxa to varying Ni concentrations; and (iv) current hypotheses for the co-evolution of Ni enzymes and seawater concentrations over Earth history.